Pivotal role of orexin signaling in the posterior paraventricular nucleus of the thalamus during the stress-induced reinstatement of oxycodone-seeking behavior.

BACKGROUND: The orexin (OX) system has received increasing interest as a potential target for treating substance use disorder. OX transmission in the posterior paraventricular nucleus of the thalamus (pPVT), an area activated by highly salient stimuli that are both reinforcing and aversive, mediates cue- and stress-induced reinstatement of reward-seeking behavior. Oral administration of suvorexant (SUV), a dual OX receptor (OXR) antagonist (DORA), selectively reduced conditioned reinstatement of oxycodone-seeking behavior and stress-induced reinstatement of alcohol-seeking behavior in dependent rats. AIMS: This study tested whether OXR blockade in the pPVT with SUV reduces oxycodone or sweetened condensed milk (SCM) seeking elicited by conditioned cues or stress. METHODS: Male Wistar rats were trained to self-administer oxycodone (0.15 mg/kg, i.v., 8 h/day) or SCM (0.1 ml, 2:1 dilution [v/v], 30 min/day). After extinction, we tested the ability of intra-pPVT SUV (15 µg/0.5 µl) to prevent reinstatement of oxycodone or SCM seeking elicited by conditioned cues or footshock stress. RESULTS: The rats acquired oxycodone and SCM self-administration, and oxycodone intake correlated with signs of physical opioid withdrawal, confirming dependence. Following extinction, the presentation of conditioned cues or footshock elicited reinstatement of oxycodone- and SCM-seeking behavior. Intra-pPVT SUV blocked stress-induced reinstatement of oxycodone seeking but not conditioned reinstatement of oxycodone or SCM seeking or stress-induced reinstatement of SCM seeking. CONCLUSIONS: The results indicate that OXR signaling in the pPVT is critical for stress-induced reinstatement of oxycodone seeking, further corroborating OXRs as treatment targets for opioid use disorder.

LY2444296, a κ-opioid receptor antagonist, selectively reduces alcohol drinking in male and female Wistar rats with a history of alcohol dependence.

Alcohol use disorder (AUD) remains a major public health concern. The dynorphin (DYN)/κ-opioid receptor (KOP) system is involved in actions of alcohol, particularly its withdrawal-associated negative affective states. This study tested the ability of LY2444296, a selective, short-acting, KOP antagonist, to decrease alcohol self-administration in dependent male and female Wistar rats at 8 h abstinence. Animals were trained to orally self-administer 10% alcohol (30 min/day for 21 sessions) and were made dependent via chronic intermittent alcohol vapor exposure for 6 weeks or exposed to air (nondependent). After 6 weeks, the effect of LY2444296 (0, 3, and 10 mg/kg, p.o.) was tested on alcohol self-administration at 8 h of abstinence. A separate cohort of rats was prepared in parallel, and their somatic withdrawal signs and alcohol self-administration were measured after LY2444296 administration at 8 h, 2 weeks, and 4 weeks abstinence. LY2444296 at 3 and 10 mg/kg significantly reduced physical signs of withdrawal in dependent rats at 8 h abstinence, only. Furthermore, 3 and 10 mg/kg selectively decreased alcohol self-administration in dependent rats at only 8 h abstinence. These results highlight the DYN/KOP system in actions of alcohol during acute abstinence, suggesting KOP antagonism could be beneficial for mitigating acute withdrawal signs and, in turn, significantly reduce excessive alcohol consumption associated with AUD.

Daily treatment with the dual orexin receptor antagonist DORA-12 during oxycodone abstinence decreases oxycodone conditioned reinstatement.

Chronic opioid use disturbs circadian rhythm and sleep, encouraging opioid use and relapse. The orexin (OX) system is recruited by opioids and regulates physiological processes including sleep. Dual OX receptor antagonists (DORAs), developed for insomnia treatment, may relieve withdrawal-associated sleep disturbances. This study investigated whether DORA-12, a recently developed DORA, reduces physiological activity disturbances during oxycodone abstinence and consequently prevents oxycodone-seeking behavior. Male and female Wistar rats were trained to intravenously self-administer oxycodone (0.15 mg/kg, 21 sessions; 8 h/session) in the presence of a contextual/discriminative stimulus (SD). The rats were subsequently housed individually (22 h/day) to monitor activity, food and water intake. They received DORA-12 (0-30 mg/kg, p.o.) after undergoing daily 1-h extinction training (14 days). After extinction, the rats were tested for oxycodone-seeking behavior elicited by the SD. Hypothalamus sections were processed to assess oxycodone- or DORA-12-associated changes to the OX cell number. In males, oxycodone-associated increases in activity during the light-phase, reinstatement, and decreases in the number of OX cells observed in the vehicle-treated group were not observed with DORA-12-treatment. Oxycodone-associated increases in light-phase food and water intake were not observed by day 14 of 3 mg/kg DORA-12-treatment and dark-phase water intake was increased across treatment days. In females, OX cell number was unaffected by oxycodone or DORA-12. Three and 30 mg/kg DORA-12 increased females' day 7 dark-phase activity and decreased reinstatement. Thirty mg/kg DORA-12 reduced oxycodone-associated increases in light-phase food and water intake. The results suggest that DORA-12 improves oxycodone-induced disruptions to physiological activities and reduces relapse.

ADX106772, an mGlu2 receptor positive allosteric modulator, selectively attenuates oxycodone taking and seeking.

Opioid abuse and overdose have risen to epidemic proportions in the United States. Oxycodone is the most abused prescription opioid. Treatments for opioid use disorder (OUD) seek to reduce vulnerability to relapse by reducing sources of reinforcement to seek drug (i.e., acute drug effects or drug withdrawal/craving). Accumulating evidence that glutamate release elicits drug-seeking behaviors has generated interest in pharmacotherapies targeting the glutamate system. Agonists and positive allosteric modulators of the metabotropic glutamate 2 (mGlu2) receptor decrease glutamate activity, reducing drug taking and seeking. The present study tested whether the mGlu2 receptor positive allosteric modulator ADX106772 reduces oxycodone self-administration and the conditioned reinstatement of oxycodone seeking without affecting behaviors directed toward a highly palatable nondrug reinforcer (sweetened condensed milk). Male Wistar rats were trained to self-administer oxycodone (0.15 mg/kg/infusion, i.v., 12 h/day) or sweetened condensed milk (SCM; diluted 2:1 v/v in H2O, orally, 30 min/day) for 13 days in the presence of a contextual/discriminative stimulus (SD), and the ability of ADX106772 (0, 0.3, 1, 3 and-10 mg/kg, s. c.) to decrease self-administration was tested. The rats then underwent extinction training, during which oxycodone, SCM, and the SD were withheld. After extinction, the ability of ADX106772 to prevent SD-induced conditioned reinstatement of oxycodone and SCM seeking was tested. ADX106772 reduced oxycodone self-administration and conditioned reinstatement without affecting SCM self-administration or conditioned reinstatement. ADX106772 reduced oxycodone taking and seeking and did not affect the motivation for the palatable conventional reinforcer, SCM, suggesting that activating mGlu2 receptors with a positive allosteric modulator is a potential approach for prescription OUD treatment.

Naltrexone blocks alcohol-induced effects on kappa-opioid receptors in the plasma membrane.

Naltrexone (NTX), a homologue of the opiate antidote naloxone, is an orally active long-acting mu-opioid receptor (MOP) antagonist used in the treatment of opiate dependence. NTX is also found to relieve craving for alcohol and is one of the few FDA-approved drugs for alcohol use disorder (AUD). Reports that NTX blocks the actions of endogenous opioids released by alcohol are not convincing, suggesting that NTX interferes with alcohol actions by affecting opioid receptors. MOP and kappa-opioid receptor (KOP) are structurally related but functionally different. MOP is mainly located in interneurons activated by enkephalins while KOP is located in longer projections activated by dynorphins. While the actions of NTX on MOP are well established, the interaction with KOP and addiction is not well understood. We used sensitive fluorescence-based methods to study the influence of alcohol on KOP and the interaction between KOP and NTX. Here we report that alcohol interacts with KOP and its environment in the plasma membrane. These interactions are affected by NTX and are exerted both on KOP directly and on the plasma membrane (lipid) structures ("off-target"). The actions of NTX are stereospecific. Selective KOP antagonists, recently in early clinical trials for major depressive disorder, block the receptor but do not show the full action profile of NTX. The therapeutic effect of NTX treatment in AUD may be due to direct actions on KOP and the receptor environment.

Strain and sex-related behavioral variability of oxycodone dependence in rats.

Over the past two decades, the escalating prescription of opioid medications for pain management has culminated in a widespread opioid epidemic, significantly impacting public health, social dynamics, and economic stability. The urgent need for improved treatments for opioid addiction necessitates a deeper understanding of its biological underpinnings, with genetic variations playing a crucial role in individual susceptibility to opioid use disorder (OUD) and influencing clinical practices. In this study, we leverage the genetic diversity of four rat strains (ACI/N, BN/NHsd, WKY/N, and F344/N) to examine the contribution of genetic factors to oxycodone metabolism and addiction-like behaviors. We used the extended access to intravenous oxycodone self-administration procedure (12 h/day, 0.15 mg/kg/injection) to comprehensively characterize oxycodone-related behaviors and pharmacokinetics. We measured escalation of oxycodone self-administration, motivation for drug consumption, tolerance to the analgesic effects of oxycodone, withdrawal-induced hyperalgesia, and oxycodone-induced respiratory depression. Additionally, we examined oxycodone-seeking behavior after four weeks of withdrawal by reintroducing the animals to environmental and cue stimuli previously associated with oxycodone self-administration. The findings revealed notable strain differences in several behavioral measures, including oxycodone metabolism. Intriguingly, BN/NHsd and WKY/N strains exhibited similar drug intake and escalation patterns but displayed significant disparities in oxycodone and oxymorphone metabolism. Minimal sex differences were observed within strains, primarily relating to oxycodone metabolism. In conclusion, this study identifies strain differences in the behavioral responses and pharmacokinetics associated with oxycodone self-administration in rats, providing a robust foundation for identifying genetic and molecular variants associated with various facets of the opioid addiction process.

Suvorexant, an FDA-approved dual orexin receptor antagonist, reduces oxycodone self-administration and conditioned reinstatement in male and female rats.

Background: The Department of Health and Human Services reports that prescription pain reliever (e.g., oxycodone) misuse was initiated by 4,400 Americans each day in 2019. Amid the opioid crisis, effective strategies to prevent and treat prescription opioid use disorder (OUD) are pressing. In preclinical models, the orexin system is recruited by drugs of abuse, and blockade of orexin receptors (OX receptors) prevents drug-seeking behavior. The present study sought to determine whether repurposing suvorexant (SUV), a dual OX receptor antagonist marketed for the treatment of insomnia, can treat two features of prescription OUD: exaggerated consumption and relapse. Methods: Male and female Wistar rats were trained to self-administer oxycodone (0.15 mg/kg, i. v., 8 h/day) in the presence of a contextual/discriminative stimulus (SD) and the ability of SUV (0-20 mg/kg, p. o.) to decrease oxycodone self-administration was tested. After self-administration testing, the rats underwent extinction training, after which we tested the ability of SUV (0 and 20 mg/kg, p. o.) to prevent reinstatement of oxycodone seeking elicited by the SD. Results: The rats acquired oxycodone self-administration and intake was correlated with the signs of physical opioid withdrawal. Additionally, females self-administered approximately twice as much oxycodone as males. Although SUV had no overall effect on oxycodone self-administration, scrutiny of the 8-h time-course revealed that 20 mg/kg SUV decreased oxycodone self-administration during the first hour in males and females. The oxycodone SD elicited strong reinstatement of oxycodone-seeking behavior that was significantly more robust in females. Suvorexant blocked oxycodone seeking in males and reduced it in females. Conclusions: These results support the targeting of OX receptors for the treatment for prescription OUD and repurposing SUV as pharmacotherapy for OUD.

Sensitivity of Hypocretin System to Chronic Alcohol Exposure: A Human and Animal Study.

Human heroin addicts and mice administered morphine for a 2 week period show a greatly increased number of hypothalamic hypocretin (Hcrt or orexin) producing neurons with a concomitant reduction in Hcrt cell size. Male rats addicted to cocaine similarly show an increased number of detectable Hcrt neurons. These findings led us to hypothesize that humans with alcohol use disorder (AUD) would show similar changes. We now report that humans with AUD have a decreased number and size of detectable Hcrt neurons. In addition, the intermingled melanin concentrating hormone (MCH) neurons are reduced in size. We saw no change in the size and number of tuberomammillary histamine neurons in AUD. Within the Hcrt/MCH neuronal field we found that microglia cell size was increased in AUD brains. In contrast, male rats with 2 week alcohol exposure, sufficient to elicit withdrawal symptoms, show no change in the number or size of Hcrt, MCH and histamine neurons, and no change in the size of microglia. The present study indicates major differences between the response of Hcrt neurons to opioids and that to alcohol in human subjects with a history of substance abuse.

Blockade of orexin receptors in the infralimbic cortex prevents stress-induced reinstatement of alcohol-seeking behaviour in alcohol-dependent rats.

BACKGROUND AND PURPOSE: A major problem managing alcohol use disorder is the high vulnerability to relapse, even after long periods of abstinence. Chronic alcohol use dysregulates stress responsivity, rendering this system hyporesponsive and making individuals vulnerable to relapse. Orexin (hypocretin) plays a role in diverse physiological processes, including stress. Orexin neurons in the hypothalamus, project to the infralimbic cortex. This study asked does infralimbic cortex orexin transmission play a significant role in stress-induced reinstatement of alcohol-seeking behaviour in alcohol-dependent rats. EXPERIMENTAL APPROACH: Male and female rats were trained to self-administer 10% alcohol (3 weeks) and then made dependent via chronic intermittent alcohol vapour exposure. Following extinction (5 days·week-1 at 8 h abstinence for 10 sessions), rats received an intra- infralimbic cortex microinfusion of the OX1/2 antagonist TCS 1102 (15 µg/0.5 µl per side) and then tested for footshock stress-induced reinstatement of alcohol seeking. In a separate cohort, orexin regulation of infralimbic cortex neuronal activity at the time of reinstatement was investigated using ex vivo electrophysiology. KEY RESULTS: TCS 1102 prevented reinstatement in dependent animals only. Moreover, Hcrtr mRNA expression in the hypothalamus and Hcrtr1/2 in the infralimbic cortex increased in alcohol-dependent animals at the time of testing. Dependence dampened basal orexin/OX receptor influence over infralimbic cortex GABAergic synapses (using TCS 1102) allow for greater stimulated orexin effects. CONCLUSION AND IMPLICATIONS: Infralimbic cortex transmission is implicate in stress-induced reinstatement of alcohol-seeking behaviour in subjects with a history of alcohol dependence and show maladaptive recruitment of infralimbic cortex transmission by alcohol dependence.

The Amygdala Noradrenergic System Is Compromised With Alcohol Use Disorder.

BACKGROUND: Alcohol use disorder (AUD) is a leading preventable cause of death. The central amygdala (CeA) is a hub for stress and AUD, while dysfunction of the noradrenaline stress system is implicated in AUD relapse. METHODS: Here, we investigated whether alcohol (ethanol) dependence and protracted withdrawal alter noradrenergic regulation of the amygdala in rodents and humans. Male adult rats were housed under control conditions, subjected to chronic intermittent ethanol vapor exposure to induce dependence, or withdrawn from chronic intermittent ethanol vapor exposure for 2 weeks, and ex vivo electrophysiology, biochemistry (catecholamine quantification by high-performance liquid chromatography), in situ hybridization, and behavioral brain-site specific pharmacology studies were performed. We also used real-time quantitative polymerase chain reaction to assess gene expression of α1B, ß1, and ß2 adrenergic receptors in human postmortem brain tissue from men diagnosed with AUD and matched control subjects. RESULTS: We found that α1 receptors potentiate CeA GABAergic (gamma-aminobutyric acidergic) transmission and drive moderate alcohol intake in control rats. In dependent rats, ß receptors disinhibit a subpopulation of CeA neurons, contributing to their excessive drinking. Withdrawal produces CeA functional recovery with no change in local noradrenaline tissue concentrations, although there are some long-lasting differences in the cellular patterns of adrenergic receptor messenger RNA expression. In addition, postmortem brain analyses reveal increased α1B receptor messenger RNA in the amygdala of humans with AUD. CONCLUSIONS: CeA adrenergic receptors are key neural substrates of AUD. Identification of these novel mechanisms that drive alcohol drinking, particularly during the alcohol-dependent state, supports ongoing new medication development for AUD.

Blockade of corticotropin-releasing factor-1 receptors in the infralimbic cortex prevents stress-induced reinstatement of alcohol seeking in male Wistar rats: Evidence of interaction between CRF1 and orexin receptor signaling.

Alcohol use dysregulates responsivity to stress, which is mediated by corticotropin-releasing factor (CRF). With repeated cycles of alcohol use, the hypothalamic-pituitary-adrenal axis becomes hyporesponsive, rendering individuals vulnerable to the reinstatement of alcohol-seeking behavior during stressful episodes. Orexin (Orx; also called hypocretin) plays a well-established role in regulating diverse physiological processes, including stress, and interacts with CRF. The infralimbic cortex (IL) is a CRF-rich region. Anatomical evidence suggests that CRF and Orx interact in this area. To test the behavioral implication of CRF and Orx transmission in the IL during the stress-induced reinstatement of alcohol-seeking behavior, male Wistar rats were trained to self-administer 10% alcohol for 3 weeks. The rats then underwent two weeks of extinction training (identical to the alcohol self-administration sessions, but alcohol was withheld). The day after the last extinction session, the rats received a bilateral intra-IL injection of the CRF1 receptor antagonist CP154,526 (0.6 µg/0.5 µl/side), the dual Orx receptor antagonist TCS1102 (15 µg/0.5 µl/side), or their combination and then were tested for the footshock stress-induced reinstatement of alcohol-seeking behavior. CP154,526 significantly prevented reinstatement, but TCS1102 did not produce such an effect. Interestingly, the co-administration of TCS1102 and CP154,526 reversed the effect of CP154,526 alone, and footshock stress induced a significant increase in Crhr1 and Hcrtr2 mRNA expression in the IL. These results demonstrate a functional interaction between Orx receptor and CRF1 receptor signaling and suggest that CRF1 receptor antagonism may ameliorate stress-induced alcohol-seeking behavior.

Linking drug and food addiction via compulsive appetite.

BACKGROUND AND PURPOSE: 'Food addiction' is the subject of intense public and research interest. However, this nosology based on neurobehavioural similarities among obese individuals, patients with eating disorders and those with substance use disorders (drug addiction) remains controversial. We thus sought to determine which aspects of disordered eating are causally linked to preclinical models of drug addiction. We hypothesized that extensive drug histories, known to cause addiction-like brain changes and drug motivation in rats, would also cause addiction-like food motivation. EXPERIMENTAL APPROACH: Rats underwent extensive cocaine, alcohol, caffeine or obesogenic diet histories and were subsequently tested for punishment-resistant food self-administration or 'compulsive appetite', as a measure of addiction-like food motivation. KEY RESULTS: Extensive cocaine and alcohol (but not caffeine) histories caused compulsive appetite that persisted long after the last drug exposure. Extensive obesogenic diet histories also caused compulsive appetite, although neither cocaine nor alcohol histories caused excess calorie intake and bodyweight during abstinence. Hence, compulsive appetite and obesity appear to be dissociable, with the former sharing common mechanisms with preclinical drug addiction models. CONCLUSION AND IMPLICATIONS: Compulsive appetite, as seen in subsets of obese individuals and patients with binge-eating disorder and bulimia nervosa (eating disorders that do not necessarily result in obesity), appears to epitomize 'food addiction'. Because different drug and obesogenic diet histories caused compulsive appetite, overlapping dysregulations in the reward circuits, which control drug and food motivation independently of energy homeostasis, may offer common therapeutic targets for treating addictive behaviours across drug addiction, eating disorders and obesity.

Alternative use of suvorexant (Belsomra®) for the prevention of alcohol drinking and seeking in rats with a history of alcohol dependence.

Alcohol use disorder (AUD) is one of the most treatment-resistant medical conditions globally. The orexin (Orx) system regulates diverse physiological processes, including stress, and is a system of interest for the development of pharmaceuticals to treat substance use disorders, particularly AUD. The present study tested the ability of the dual orexin receptor antagonist suvorexant (SUV), marketed by Merck as Belsomra®, for the treatment of insomnia, to decrease alcohol self-administration and the stress-induced reinstatement of alcohol-seeking behavior in male Wistar rats with a history of alcohol dependence. Rats were trained to orally self-administer 10% alcohol (30 min/day for 3 weeks) and were either made dependent via chronic intermittent alcohol vapor exposure (14 h ON, 10 h OFF) for 6 weeks or exposed to air (non-dependent). Starting on week 7, the effect of SUV (0-20 mg/kg, p.o.) was tested on alcohol self-administration at acute abstinence (8 h after vapor was turned OFF) twice weekly. A separate cohort of rats that were prepared in parallel was removed from alcohol vapor exposure and then subjected to extinction training for 14 sessions. Once extinction was achieved, the rats received SUV (0 and 5 mg/kg, p.o.) and were tested for the footshock stress-induced reinstatement of alcohol-seeking behavior. Suvorexant at 5, 10, and 20 mg/kg selectively decreased alcohol intake in dependent rats. Furthermore, 5 mg/kg SUV prevented the stress-induced reinstatement of alcohol-seeking behavior in dependent rats only. These results underscore the significance of targeting the Orx system for the treatment of substance use disorders generally and suggest that repurposing SUV could be an alternative approach for the treatment of AUD.

Blockade of corticotropin-releasing factor receptor 1 in the central amygdala prevents cocaine-seeking behaviour induced by orexin-A administered to the posterior paraventricular nucleus of the thalamus in male rats.

Background: Orexin-A (OrxA) administration in the posterior paraventricular nucleus of the thalamus (pPVT) reinstates extinguished cocaine-seeking behaviour following extended access to the drug (a model of dependence). The pPVT receives and integrates information associated with emotionally salient events and sends excitatory inputs to brain regions involved in the expression of emotional states, such as those driving cocaine-seeking behaviour (i.e., the nucleus accumbens, the central nucleus of the amygdala [CeA], the basolateral amygdala, the bed nucleus of the stria terminalis [BNST] and the prefrontal cortex). Methods: We monitored the activation pattern of these regions (measured by Fos) during cocaine-seeking induced by OrxA administered to the pPVT. The BNST and CeA emerged as being selectively activated. To test whether the functionality of these regions was pivotal during OrxA-induced cocaine-seeking behaviour, we transiently inactivated these regions concomitantly with OrxA administration to the pPVT. We then tested the participation of corticotropin-releasing factor receptors (CRF1) in the CeA during OrxA-induced cocaine-seeking using the CRF1 antagonist CP154526. Results: We observed selective activation of the CeA and BNST during cocaine-seeking induced by OrxA administered to the pPVT, but only transient inactivation of the CeA prevented cocaine-seeking behaviour. Administration of CP154526 to the CeA prevented OrxAinduced cocaine-seeking behaviour. Limitations: The use of only male rats could have been a limitation. Other limitations could have been the use of an indirect approach to test the hypothesis that administration of OrxA to the pPVT drives cocaine-seeking via CRF1 signalling in the CeA, and a lack of analysis of the participation of CeA subregions. Conclusion: Cocaine-seeking behaviour induced by OrxA administered to the pPVT is driven by activation of the CeA via CRF1 signalling.

Cocaine-Seeking Behavior Induced by Orexin A Administration in the Posterior Paraventricular Nucleus of the Thalamus Is Not Long-Lasting: Neuroadaptation of the Orexin System During Cocaine Abstinence.

Hypothalamic orexin (Orx) projections to the paraventricular nucleus of the thalamus (PVT) have received growing interest because of their role in drug-seeking behavior. Using an established model of cocaine dependence (i.e., long access [LgA] to cocaine), we previously showed that OrxA injections in the posterior PVT (pPVT) reinstated extinguished cocaine-seeking behavior in rats after an intermediate period of abstinence (2-3 weeks). Considering the long-lasting nature of drug-seeking behavior, the present study examined whether the priming effect of intra-pPVT OrxA administration was preserved after a period of protracted abstinence (4-5 weeks) in rats that self-administered cocaine under LgA conditions. Furthermore, to better understand whether a history of cocaine dependence affects the Orx system-particularly the hypothalamic Orx↔pPVT connection-the number of Orx-expressing cells in the lateral hypothalamus (LH), dorsomedial hypothalamus (DMH), and perifornical area (PFA) and number of orexin receptor 1 (OrxR1)- and OrxR2-expressing cells in the pPVT were quantified. Orexin A administration in the pPVT induced cocaine-seeking behavior after intermediate abstinence, as reported previously. At protracted abstinence, however, the priming effect of OrxA was absent. A higher number of cells that expressed Orx was observed in the LH/DMH/PFA at both intermediate and protracted abstinence. In the pPVT, the number of OrxR2-expressing cells was significantly higher only at intermediate abstinence, with no changes in the number of OrxR1-expressing cells. These data build on our previous findings that the hypothalamic Orx↔pPVT connection is strongly recruited shortly after cocaine abstinence and demonstrate that the priming effect of OrxA is not long lasting. Furthermore, these findings suggest that throughout abstinence, the Orx↔pPVT connection undergoes neuroadaptive changes, reflected by alterations of the number of OrxR2-expressing cells in the pPVT.

Selective inhibition of monoacylglycerol lipase is associated with passive coping behavior and attenuation of stress-induced dopamine release in the medial prefrontal cortex.

The endocannabinoid system is involved in the regulation of the stress response, but the relative contribution of N-arachidonoylethanolamine (AEA) and 2-arachidonoylglycerol (2-AG) and their mechanisms have to be elucidated. In this study, we compared the effects of the pharmacological inhibition of the two major endocannabinoid-degrading enzymes [fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL) for AEA and 2-AG, respectively] on stress-coping [forced swim test (FST) and tail suspension test (TST)] and anxiety-like [elevated-plus maze (EPM) and light-dark test (LDT)] behaviors in wild-type and FAAH knockout mice. In vivo microdialysis estimated the effects of FAAH and MAGL inhibition on dopamine (DA) and serotonin (5-HT) levels in the medial prefrontal cortex (mPFC) during an FST. Mice were treated with PF-3845 (FAAH inhibitor), JZL184 (MAGL inhibitor), JZL195 (dual FAAH/MAGL inhibitor) or vehicle. Our data showed that PF-3845 increased latency to immobility and decreased total immobility time in FST, but no effects were observed in TST compared with vehicle-treated wild-type mice. By contrast, JZL184 decreased latency and increased immobility in TST and FST. JZL195 in wild-type mice and JZL184 in FAAH knockout mice reproduced the same passive coping behaviors as JZL184 in wild-type mice in TST and FST. In the microdialysis experiment, FST was associated with increased DA and 5-HT levels in the mPFC. However, JZL184-treated wild-type mice displayed a significant attenuation of forced swim stress-induced DA release compared with vehicle-treated wild-type mice and PF-3845-treated wild-type mice. Finally, FAAH and/or MAGL inhibitors induced robust and consistent anxiolytic-like effects in EPM and LDT. These results suggested differences between FAAH and MAGL inhibition in stress-coping behaviors. Notably, MAGL inhibition induced a consistent avoidant coping behavior and attenuated the stress-induced mPFC DA response in FST. However, more investigation is needed to elucidate the functional association between DA and 2-AG signaling pathways, and the molecular mechanism in the regulation of passive coping strategies during inescapable stress.

Understanding the Role of Orexin Neuropeptides in Drug Addiction: Preclinical Studies and Translational Value.

Orexins (also known as hypocretins) are neuropeptides that participate in the regulation of energy metabolism, homeostasis, sleep, feeding, stress responses, arousal, and reward. Particularly relevant to the scope of the present review is the involvement of the orexin system in brain mechanisms that regulate motivation, especially highly motivated behavior, arousal, and stress, making it an ideal target for studying addiction and discovering treatments. Drug abuse and misuse are thought to induce maladaptive changes in the orexin system, and these changes might promote and maintain uncontrolled drug intake and contribute to relapse. Dysfunctional changes in this neuropeptidergic system that are caused by drug use might also be responsible for alterations of feeding behavior and the sleep-wake cycle that are commonly disrupted in subjects with substance use disorder. Drug addiction has often been associated with an increase in activity of the orexin system, suggesting that orexin receptor antagonists may be a promising pharmacological treatment for substance use disorder. Substantial evidence has shown that single orexin receptor antagonists that are specific to either orexin receptor 1 or 2 can be beneficial against drug intake and relapse. Interest in the efficacy of dual orexin receptor antagonists, which were primarily developed to treat insomnia, has grown in the field of drug addiction. Treatments that target the orexin system may be a promising strategy to reduce drug intake, mitigate relapse vulnerability, and restore "normal" physiological functions, including feeding and sleep. The present review discusses preclinical and clinical evidence of the involvement of orexins in drug addiction and possible beneficial pharmacotherapeutic effects of orexin receptor antagonists to treat substance use disorder.

Blockade of Orexin Receptors in the Posterior Paraventricular Nucleus of the Thalamus Prevents Stress-Induced Reinstatement of Reward-Seeking Behavior in Rats With a History of Ethanol Dependence.

Neural systems involved in processing natural rewards and drugs of abuse overlap and exposure to drugs of abuse induce neuroadaptations that can cause compulsive-like behavior. For example, the recruitment of the orexin (Orx) system by drugs of abuse has been proposed to induce neuroadaptations that in turn alter its function, reflected by maladaptive, compulsive, and addictive behavior. Orexin neurons project to the paraventricular nucleus of the thalamus (PVT)-particularly the posterior part (pPVT), a structure that plays a key role in stress regulation. This study investigated whether Orx transmission in the pPVT plays a role in stress-induced reinstatement of reward-seeking behavior toward ethanol (EtOH) and a highly palatable food reward [sweetened condensed milk (SCM)] in rats and whether this role changes with EtOH dependence. After being trained to orally self-administer EtOH or SCM, the rats were made dependent (EtOHD and SCMD) by chronic intermittent EtOH vapor exposure. The control nondependent groups (EtOHND and SCMND) were exposed to air. Following extinction, the rats were tested for stress-induced reinstatement of EtOH- and SCM-seeking behavior. Stress reinstated EtOH- and SCM-seeking behavior in all groups (EtOHD/ND and SCMD/ND). Administration of the dual Orx receptor (OrxR) antagonist TCS1102 (15 µg) in the pPVT prevented stress-induced reinstatement only in dependent rats (EtOHD and SCMD). In parallel, the qPCR analysis showed that Orx mRNA expression in the hypothalamus and OrxR1/R2 mRNA expression in the pPVT were increased at the time of testing in the EtOHD and SCMD groups. These results are the first to implicate Orx transmission in the pPVT in the stress-induced reinstatement of reward-seeking behavior in EtOH dependent rats and indicate the maladaptive recruitment of Orx transmission in the pPVT by EtOH dependence.

COX-2 Inhibition Antagonizes Intra-Accumbens 2-Arachidonoylglycerol-Mediated Reduction in Ethanol Self-Administration in Rats.

BACKGROUND: Ethanol (EtOH) self-administration is particularly sensitive to the modulation of CB1 signaling in the nucleus accumbens (NAc) shell, and EtOH consumption increases extracellular levels of the endogenous cannabinoid CB1 receptor agonist 2-arachidonoyl glycerol (2-AG) in this brain region. Stimulation of CB1 receptor with agonists increases EtOH consumption, suggesting that EtOH-induced increases in 2-AG might sustain motivation for EtOH intake. METHODS: In order to further explore this hypothesis, we analyzed the alterations in operant EtOH self-administration induced by intra-NAc shell infusions of 2-AG itself, the CB1 inverse agonist SR141716A, the 2-AG clearance inhibitor URB602, anandamide, and the cyclooxygenase-2 (COX-2) inhibitor nimesulide. RESULTS: Surprisingly, self-administration of 10% EtOH was dose-dependently reduced by either intra-NAc shell SR141716A or 2-AG infusions. Similar effects were found by intra-NAc shell infusions of URB602, suggesting again a role for accumbal 2-AG on the modulation of EtOH intake. Intra-NAc shell anandamide did not alter EtOH self-administration, pointing to a specific role for 2-AG in the modulation of EtOH self-administration. Finally, the inhibitory effect of intra-NAc shell 2-AG on EtOH intake was significantly reversed by pretreatment with nimesulide, suggesting that oxidative metabolites of 2-AG might mediate these inhibitory effects on operant self-administration. CONCLUSIONS: We propose that 2-AG signaling in the NAc exerts an inhibitory influence on EtOH consumption through a non-CB1 receptor mechanism involving the COX-2 pathway.

Targeting the Orexin System for Prescription Opioid Use Disorder.

Prescription opioids are potent analgesics that are used for clinical pain management. However, the nonmedical use of these medications has emerged as a major concern because of dramatic increases in abuse and overdose. Therefore, effective strategies to prevent prescription opioid use disorder are urgently needed. The orexin system has been implicated in the regulation of motivation, arousal, and stress, making this system a promising target for the treatment of substance use disorder. This review discusses recent preclinical studies that suggest that orexin receptor blockade could be beneficial for the treatment of prescription opioid use disorder.